Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	$ 6,888	$ 5,676	$ 6,122	$ 5,739
Provision (credit) for loan losses	800	150	1,600	350
Recoveries of amounts charged off	6	0	29	10
Balance, before amounts charged off	7,694	5,826	7,751	6,099
Amounts charged off	(3)	(18)	(60)	(291)
Balance, end of period	7,691	5,808	7,691	5,808
Residential real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	1,590	1,396	1,392	1,368
Provision (credit) for loan losses	5	31	180	116
Recoveries of amounts charged off	5	0	28	5
Balance, before amounts charged off	1,600	1,427	1,600	1,489
Amounts charged off	0	(18)	0	(80)
Balance, end of period	1,600	1,409	1,600	1,409
Construction real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	594	646	774	617
Provision (credit) for loan losses	75	43	(105)	72
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	669	689	669	689
Amounts charged off	0	0	0	0
Balance, end of period	669	689	669	689
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	3,832	3,011	3,178	2,933
Provision (credit) for loan losses	313	156	1,021	234
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	4,145	3,167	4,199	3,167
Amounts charged off	0	0	(54)	0
Balance, end of period	4,145	3,167	4,145	3,167
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	491	313	394	354
Provision (credit) for loan losses	1	13	98	171
Recoveries of amounts charged off	0	0	0	1
Balance, before amounts charged off	492	326	492	526
Amounts charged off	0	0	0	(200)
Balance, end of period	492	326	492	326
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	23	23	23	23
Provision (credit) for loan losses	(4)	2	(1)	9
Recoveries of amounts charged off	1	0	1	4
Balance, before amounts charged off	20	25	23	36
Amounts charged off	(3)	0	(6)	(11)
Balance, end of period	17	25	17	25
Municipal
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	79	33	76	82
Provision (credit) for loan losses	133	45	136	(4)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	212	78	212	78
Amounts charged off	0	0	0	0
Balance, end of period	212	78	212	78
Unallocated
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance, beginning of period	279	254	285	362
Provision (credit) for loan losses	277	(140)	271	(248)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	556	114	556	114
Amounts charged off	0	0	0	0
Balance, end of period	$ 556	$ 114	$ 556	$ 114